Some automotive fuels may exhibit rapid evaporation in response to diurnal variations in ambient temperature. Emissions resulting from such vapors may be reduced in automotive applications via evaporative emission control systems (EVAP). The EVAP systems include a fuel vapor storage canister containing adsorbent, such as carbon, that traps those fuel vapors and feeds them back to the vehicle's engine for combustion during canister purging operations, thus, reducing evaporative emissions from the vehicle and improving fuel economy.
To counter the effects of diurnal variations in temperature and other vehicle operating conditions which may increase a rate at which fuel vapors are transferred from the fuel tank to the fuel vapor storage canister, some fuel systems may include sealed fuel tanks. Such fuel tanks may be referred to as non-integrated refueling canister-only systems (NIRCOS), and because they are sealed, may build up high pressures. A tank pressure control valve (TPCV) may be duty cycled to relieve such pressure under particular vehicle operating conditions. However, duty cycling the TPCV may contribute to degradation, which may be exacerbated if the TPCV is commanded to open and/or close when there is a high pressure difference across the valve. Thus, pressure pulsations stemming from engine operation may be a contributor to TPCV degradation, and may contribute to undesirable NVH issues.
The inventors herein have recognized the above-mentioned issues, and have developed systems and methods to at least partially address them. In one example, a method comprises depressurizing a fuel tank of a vehicle by duty cycling a tank pressure control valve and routing fuel vapors from the fuel tank to an engine, while controlling a timing of opening and closing events of the tank pressure control valve to coincide with pressure differences across the tank pressure control valve less than a threshold pressure difference in terms of pressure oscillations across the tank pressure control valve. In this way, degradation of the CPV and issues related to NVH may be reduced or avoided.
In one example of the method, the method may further comprise learning a fuel vapor concentration stemming from the fuel tank during depressurizing the fuel tank, and maintaining a desired engine air-fuel ratio while depressurizing the fuel tank based on the learned fuel vapor concentration. In such an example, the method may further comprise sequentially increasing the duty cycle of the tank pressure control valve while controlling the timing of the opening and the closing events of the tank pressure control valve to coincide with the pressure differences across the tank pressure control valve less than the threshold pressure difference in terms of pressure oscillations across the tank pressure control valve.
In such a method, the method may further comprise depressurizing the fuel tank in response to a pressure in the fuel tank rising above a first threshold pressure, and may further comprise depressurizing the fuel tank until a second threshold pressure is reached in the fuel tank.
In such a method, the method may further comprise commanding fully open a canister purge valve positioned in a purge conduit that couples the engine to a fuel vapor storage canister. Such a method may further comprise depressurizing the fuel tank under conditions where a canister load is less than a threshold canister load.
In another example of such a method, the method may further comprise determining the pressure oscillations across the tank pressure control during depressurizing the fuel tank. In response to changes in a frequency, a phase and/or an amplitude of the pressure oscillations, the method may include adjusting the timing of the opening and the closing events of the tank pressure control valve to maintain the opening and the closing events to coincide with the pressure differences across the tank pressure control valve less than the threshold pressure difference in terms of pressure oscillations across the tank pressure control valve.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.